It used to be that string theorists would respond to arguments that string theory predicted nothing with the claim that it predicted supersymmetry. For example, in an interview with Witten done for the PBS Elegant Universe series, one sees:
NOVA: It seems like the standard criticism of string theory is that it isn’t testable. How do you respond to that criticism?
Witten: One very important aspect of string theory is definitely testable. That was the prediction of supersymmetry, which emerged from string theory in the early ’70s. Experimentalists are still trying to test it. It hasn’t been proved that supersymmetry is right. But there is a very precise relationship among the interaction rates of different kinds of particles which follows from supersymmetry and which has been tested successfully. Because of that and a variety of other clues, many physicists do suspect that our present decade is the decade when supersymmetry will be discovered. Supersymmetry is a very big prediction; it would be interesting to delve into history and try to see any theory that ever made as big a prediction as that.
Of course the problem with this was always that supersymmetry had to be broken somehow, and string theory said nothing about how to break it, not even the scale of the breaking. Back in 2004 when the anthropic landscape business began, Susskind was enthusiastic about the idea that it could be used to predict the scale of supersymmetry breaking, and Michael Douglas started working on computations counting string vacua that were supposed to say something about this (I’ve followed this story in several blog postings, an early one was here). The bottom line quickly became clear: a host of problems make this impossible, string theory remains incapable of predicting anything about this.
Today at the Simons Center, Douglas gave a talk entitled Does String Theory Predict Low Energy Supersymmetry? (video available here), and not surprisingly the conclusion is still that string theory predicts nothing about this. Amusingly, someone in the audience took exception to Douglas saying that string theory doesn’t now make predictions, and one gets to hear Douglas try and explain to his fellow string theorist what a real prediction is. The video quality is great, but the sound doesn’t work so well when two people are loudly trying to talk over each other.
This particular talk was held indoors, for a report on what the outdoor ones have been like, see here.
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